A variety of carburetors are used to supply fuel to general purpose two-cycle engines. These engines are typically used as a source of motive power in small vehicles and portable machinery used in agriculture and forestry, etc. One particular type of carburetor has a structure in which a cylindrical throttle valve is Installed crosswise in the air intake passage of the carburetor main body. The cylindrical throttle valve includes a throttle orifice and a metering valve, and is caused to move along its own central axial line while rotating in accordance with the operation of the accelerator pedal. The throttle valve controls the air flow rate by varying the degree of overlap of the throttle orifice with the air intake passage, and controls the fuel flow rate by varying the depth of insertion of the metering needle into the fuel nozzle. See, for example, in Japanese Patent Application Kokai No. Sho 58-101253 and Japanese Utility Model Application Kokai No. Sho 62-20158.
In a state where the accelerator pedal is released, the throttle valve is placed in a position which supplies the air and fuel necessary for idle revolution of the engine. From the idle position, the throttle valve is moved in accordance with the operation of the accelerator to increase the amounts of air and fuel.
As is universally known, the starting of an engine, especially starting at low temperatures, requires larger amounts of air and fuel than ordinary idling. Accordingly, in the aforementioned rotary throttle valve type carburetor, the throttle valve in the idle position is caused to move slightly by the operation of the accelerator so that the amounts of air and fuel are increased.
However, manual operation of the accelerator tends to result in excesses or insufficiencies in the amount of air and fuel supplied to the engine. Such excesses or insufficiencies will commonly cause the engine to fail to start. Thus, skill is required in order to obtain reliable starting. In addition, manual operation of the accelerator is extremely inconvenient because the state of operation of the accelerator must be maintained such that the throttle valve is held in an "increase" position until engine warm-up is completed.